The present invention relates to a continuous process for the polymerization of polyamides that may be conducted under operating conditions to ultimately yield multiple phases. More particularly, the present invention relates to a controlled process for polyamide formation utilizing less stringent conditions of temperature and pressure otherwise required in conventional, phase-limited processes.
Conventional techniques for hydrolytic polymerization of polyamides usually employ an aqueous solution of ingredients. Polymerization is accomplished by the gradual removal of the water from the mixture at elevated pressures by the continuous application of heat (and a subsequent increase in the temperature of the reaction medium). In this manner the majority of the water is removed and the temperature of the reaction medium is above the melting point of the polyamide. The reaction pathsxe2x80x94defined as combinations of temperature and pressure conditions either in time for a batch process or at different reaction zones for a continuous processxe2x80x94are chosen in such a way that the reaction mixture is maintained in a liquid phase. This requirement to avoid any liquid-solid phase separation usually implies operating at significantly elevated pressures and correspondingly high temperatures in order to remove the water from the reaction mixture during the early stages of the polymerization, usually in excess of 300 to 400 psig for reaction mixtures containing terephthalic acid, such as PA-6T/66. Furthermore, removal of the remaining water in the later stages of polymerization by gradual reduction of pressure and increasing temperature above the melting point of the polymer requires relatively long times due to heat transfer limitations. One disadvantage of polymerization under these conditions is the resultant high degree of degradation reactions and products which diminishes the usefulness of the final polymer product.
Altogether conventional techniques such as those described above and associated with the polymerization and formation of polyamides have a number of constraints. Of significant interest, the process for conversion of the monomers to low molecular weight polymer is only accomplished by operating at conditions of pressure, temperature and polymer concentration in water corresponding to the single phase region outside the solid polymer melting phase boundary. Moreover, this operating condition must be maintained from beginning to completion of the polymerization. While processes can be based on maintaining operating conditions outside the liquid polymer freezing line during the early molecular mass building stage of polymerization, the region between the liquid polymer freezing line and the solid polymer melting line is then crossed rapidly by aggressively applying energy to the polymerizing medium.
The effect of the occurrence of these types of phases is traditionally considered disadvantageous to sustaining heat transfer efficiency and vessel operating lifetime. To compensate for this effect, those of skilled in the art typically conduct early stage polymerization of polyamide systems based upon terephthalic acid, such as PA-6T/66, at elevated conditions of pressure and temperature so that the reaction proceeds above the solid polymer melting phase boundary. See for example, JP 7138366. Alternatively, two step semi-continuous processes have been employed for the polymerization of these polymers. Such approaches first require the formation of a low molecular weight polymer at high pressures and temperatures and later isolated either in solid or liquid form from the early stages of the polymerization. Further molecular weight build-up is achieved through subsequent processing using operating conditions which allow for rapid heating of the low molecular weight polymer above its melting point in high shear fields and generation of mechanical heat, like twin screw extruders.
However, there are numerous deleterious consequences in choosing to operate at conditions of elevated temperatures and pressure early in the polymerization. Most particularly, high temperatures prompt the early inception of degradation reactions, which have the effect of diminishing the usefulness of the final polymer product. An example is the amidine branching equilibrium associated with polymerization involving aromatic diacids. Further, the influence of pressure on fluid physical properties such as vapour phase density and vapour/liquid interfacial tension may be detrimental to achieving good heat transfer performance. Moreover, such approaches have additional production costs associated with the isolation and re-melt of the oligomer for the two step process, and pose challenges in the handling of powders. Even if the oligomer is kept in molten form there are a number of difficulties in limiting the degradation and contamination of materials, typically associated with oligomer-vapor separation chambers run at excessively high temperatures.
There is a need for a process for the production of polyamides that avoids the longstanding requirement to operate at conditions in which deleterious polymerization side reactions, and with their attendant adverse heat and mass transfer physics, are associated. With such a process, product of enhanced quality will be obtained. Improvements in capital costs and operating productivity are also benefits to such a process.
A continuous hydrolytic polymerization process for the formation of polyamides or copolyamides is provided, comprising:
(a) polymerizing an aqueous salt mixture of diacids and diamines suitable to form a polyamide or a copolyamide under conditions of temperature and pressure sufficient to ultimately yield a reaction mixture in multiple phases, but for a time sufficient to avoid phase separation;
(b) transferring heat into said reaction mixture while reducing pressure of said reaction mixture sufficient to remove the water therefrom without solidification thereof; and
(c) further polymerizing said reaction mixture having the water removed and until a copolymerized product of desired molecular weight is achieved.
The ultimate mixture in step (a) may be in a multi-phase of any of solid, liquid and vapor products. Without intending to limit the generality of the foregoing components of the aqueous salt mixture in (a) above, it is understood by those having skill in the art that terephthalic acid and hexamethylene diamine and at least one of adipic acid or 2-methylpentamethylene diamine (hereinafter xe2x80x9cMPMDxe2x80x9d) are commonly used.